544 



THE SPINAL CORD AND ITS MEMBRANES. 



Characters 

 of roots. 



Some sets 

 of fibrils 

 join ; 



posterior 

 root larger, 



proportion- 

 ally largest 

 in neck. 



Boots are 

 largest for 

 nerves of 

 limbs. 



Oblique in 

 their course, 



most so 

 inferiorly, 



where they 

 form cauda 

 equina. 



Length in- 

 creases from 

 above 

 downwards. 



Union of 

 the roots 

 in inter- 

 vertebral 

 foramen. 



except in 

 first two 

 cervical, 



the sacral, 



and coc- 



c-ygeal 



nerves. 



Situation of 

 ganglia. 



Exceptions 

 in cervical, 



Characters of the roots. Besides variations in the relative size of 

 the two roots, the following characters are to be noted : 



Union of the fibrils. The fibrils of contiguous anterior roots may 

 be intermingled, and the fibrils of the neighbouring posterior roots 

 may be connected in a like manner ; but the anterior is never 

 mixed with the posterior root. 



Size of the roots to each other. The posterior root is larger than 

 the anterior, except in the suboccipital nerve ; and the number of 

 the filaments is also greater. Farther, the posterior is propor- 

 tionally larger in the cervical than in any other group ; in the 

 dorsal nerves there is but a very slight difference in favour of the 

 hinder root. 



Size of both roots along the cord. Both roots are larger where the 

 nerves for the limbs arise than at any other part of the cord ; and 

 they are largest in the nerves to the lower limbs. They are 

 smallest at the lower extremity of the cord. 



Direction and length of the roots. As the apertures of transmission 

 from the spinal canal are not generally opposite the place of origin 

 of the nerves, the roots are for the most part directed obliquely. 

 This obliquity increases from above downwards ; for in the upper 

 cervical nerves the roots are horizontal ; but in the lumbar and 

 sacral nerves they have a vertical direction around the filurn 

 terminate (fig. 197) ; and the bundle of long nerve-roots descending 

 from the end of the spinal cord, from its resemblance to a horse's 

 tail, is named the Cauda Equina. 



The length of the roots increases in proportion to their obliquity. 

 Thus, the distance between the origin and the place of exit of the 

 roots of the lowest cervical nerve equals the depth of one vertebra ; 

 in the lowest dorsal nerve it amounts to the depth of two vertebrae ; 

 and in the lumbar and sacral nerves each succeeding root becomes 

 nearly a vertebra longer, for the cord does not reach beyond the 

 first lumbar vertebra. 



Place of union of the roots. Commonly the roots unite as before 

 stated in the intervertebral foramina ; and the trunk of the nerve 

 bifurcates at the same spot into anterior and posterior primary 

 branches (fig. 196, d and e). But deviations from this arrangement 

 are found at the upper and lower ends of the spinal column in the 

 following nerves. 



The roots of the first two cervical nerves join on the neural 

 arches of the corresponding vertebrae ; and the anterior and posterior 

 primary branches diverge from the trunks in that situation. 



In the sacral nerves the union of the roots takes place within 

 the spinal canal ; and the primary branches of the nerves issue by 

 the apertures on the front and back of the sacrum. 



The roots of the coccygeal nerve are also united in the spinal 

 canal ; and the anterior and posterior branches of its trunk escape 

 by the lower aperture of that canal. 



Situation of the ganglia. The ganglia are placed commonly in the 

 intervertebral foramina, but where the position of these apertures 

 is irregular, as at the upper and lower extremities of the spinal 



